Multitap electrical element and method of making same



June 13, 1961 MULTIT A. s. LOUIS 2,988,721

AP ELECTRICAL ELEMENT AND METHOD OF MAKING SAME Filed Aug. '7, 1959 FIG. 4

Z; FIG. 5

14 uqlllll-lllll NQ-l IIIIHN INVENTOR.

ARNOLD s. LOUI'S i 2,988,721 MULTITAP ELECTRICAL ELEMENT AND METHOD OF MAKING SAME Arnold S. Louis, 87 Southgate Ave., Hastings on Hudson, NY. Filed Aug. 7, 1959, Ser. No. 832,329 4 Claims. (Cl. 338-462) This invention relates to improved multiple tap elec trioally conductive devices, an improved method of making a plurality of small dimension electrically conductive taps to electrical elements and in particular, but not limited thereto, to multitap potentiometers and to improved switching devices.

One application of this invention is in the manufacture of multitap variable resistance devices employing a resistance element traversed by a movable contact. A need exists for an improved method of making fixed contact -to the resistance element. Since the contact or tap is usually of highly conductive metal so as to provide minimum resistance, shorting out of the resistance element in the region of contact will occur. It may be appreciated that such shorting will be detrimental to the linearity of the resistance element. This effect is of particular concern where the resistance element is employed as a potentiometer type sensing element in a control system for producing an output voltage proportional to a condition being sensed. It is essential that, in the normal direction of current flow through the resistance element, the width of the tap be a minimum so as to maintain uniform the linearity of the resistance element. In the past printed circuit techniques have been employed to obtain taps of minimal width. However, even with the use of advanced photo-chemical printed circuit techniques, it is impractical to lay down a conductive line having a width of less than 0.005". It will be appreciated that one hundred tap members each but 0.00 wide, connected to even a relatively long resistance element, say one having an overall length of 10", will occupy a section of resistance element comprising 0.500" or approximately 5% of the active length. On the other hand, the device of this invention may be made with conductive strips having a width as measured in the direction of current flow through the resistance element, of but 0.001 which corresponds to but 1% of the active length with a corresponding improvement in accuracy. The conductive member is of substantial cross-sectional area so as to provide adequate current carrying capacity with minimum contact Width as measured in the direction of current flow.

An unexpected advantage of this invention is that it provides an improved potentiometer device by virtue of the multiplicity of highly conductive paths of small dimension periodically spaced at right angles to the normal path of current flow through the resistance element. The highly conductive paths insure distribution of the current flow across the path.

Briefly stated, in carrying out this invention, there is provided a plurality of spaced insulator plastic rib mem-v bers supported by an insulator plastic base. A thin layer of a highly conductive material, such as a metal, is then applied to the rib members to form individual conductive vanes supported by the rib members. The supporting ribs together with their conductive coatings are then encapsulated in a plastic composition to provide a monolithic body. In a succeeding operation the monolithic body is machined so as to expose the edge of the conductive vanes. Depending on the end product desired, the resulting element is then provided with a resistive track or conductive portions in contact with the conductive vanes.

Accordingly, it is one object of this invention to provide an improved multitap potentiometer.

It is another object of this invention to provide a novel switching device.

It is still a further object of this invention to provide a method of making a multitap potentiometer having accurately positioned electrically conductive taps.

It is a different object to provide thin, mechanically secure, electrically conductive connecting means.

It is still a different object of this invention to provide a potential equalizing means for a resistance element.

It is an object of this invention to provide a method for accurately locating very thin electrically conductive elements within a molded plastic body.

Still other objects and advantages of this invention will be pointed out with particularity and still others will become apparent as the following description proceeds taken in conjunction with the accompanying drawings.

In the drawings:

FIGURE 1 shows in plan a molded plastic base provided with a plurality of rib members.

FIGURE 2 shows, in elevation, a cross-section of the member of FIGURE 1 taken along line 22.

FIGURE 3 shows in elevation a cross-section of the base member of FIGURE 1 provided with an electrically conductive coating on the rib members and upper surface.

FIGURES 4 and 5 shown in plan and in a partially sectioned elevation respectively, a mold employed in carrying out the invention.

FIGURE 6 shows, in elevation and partially sectioned, the mold member of FIGURE 5 with the molded base member of FIGURE I, inserted therein and comolded to an encapsulating plastic body.

FIGURE 7 is a bottom view of the molded structure formed in the mold shown in FIGURE 6 with encapsulated metal members shown by dashed lines.

FIGURE 8 shows the structure of FIGURE 7 after the surface has been machined to expose an edge of the encapsulated metal members.

FIGURE 9 shows, in plan, a commutator element.

FIGURE 10 is a section taken in elevation along line 10-10 of FIGURE 9.

FIGURE 11 shows pictorially, partially broken away,

the device of this invention embodied in a potentiometen In carrying out the invention, an insulator plastic, such as for example, asbestos filled phenol formaldehyde resin,

is molded to produce a base member 11 having a plurality of ribs HA-12L. The member 11 is provided with a pair of positioning holes 13. As part of the process, it is desired to provide a number of isolated thin metal members each having a thickness of from about 0.001 to about 0.002". A simple method of forming the members is by electroplating. A conductive material is coated on faces 14A-14L, corresponding to one face of each of the ribs 12A-12L respectively, surface 16, and hub segments 17. The resultant conductive coating serves as the cathode in an electroplating operation and is electroplated with copper or other suitable metal. The function of the portion of the coating covering portion 16 and the hub The conductive material may be applied by many techniques. However, it has been found that the simplest and most satisfactory method is to paint the surfaces with.

a silver base lacquer, such as for example, Conductive Silver No. 4 132 as supplied by E. I. du Pont De Nemours' & Co. (Inc.) and then electroplate a layer of copper of about 0.001" to 0.002" in thickness onto the lacquer; Conventional electroplating techniques may be employed. Y

In FIGURE 3, in a cross section taken along line 3-3, there is shown the device after the electroplating operation. The portion of the metal layer covering the upper surface 16, hub segments 17-, and face 14D of rib 12D are identified by numerals .18, 119, and 20 respectively. It will be noted that the face of .rib 12K, shown in the sectional view of FIGURE 3, is uncoatedwhereas face 14D0f rib 12D is coated.

A cavity mold 22, shown in FIGURES 4 and 5, is used to encapsulate the metal coated base 11. The coated base .11 is inserted into mold 22 and precisely located therein by pins 23-mating with holes 13 and a mold collar 24' is positioned as shown in FIGURE 6. Additional thermosetting molding resin, in powder form, is then charged in the mold whereupon piston 26 is inserted in the collar 24. Utilizing normal molding pressures and temperatures for the resin, the particles of plastic are then molded to form a solid body 29 comolded to the base member 11.

Upon completion of the normal molding cycle, the mold is opened and the resultant comolding 27 is removed.

The molding 27 is shown in FIGURE 7, attention being directed to skirt portion 28 which provides a convenient guide for a subsequent operation. The embedded metal layer is shown by dashed lines 19 and 20.

Both faces of molding 27 are then machined, preferably by a surface grinder. The machining operation removes from one face the skirt portion 28 as well as the electrically conductive layer 18 originally placed upon surface 16 and from the other face the portion of base 11 from which ribs '12 extend.

In FIGURE 8 the result of the grinding operation is shown. There will be seen that there is now exposed an edge of the plurality of metal portions 20 embedded in plastic comolding 27. The edges of segments 19 are likewise exposed. The opposite face of the member will appear identical (except for dimples 30) as the machining operation exposes the metal portions 19 and 20. It is to be noted that segments 19 are not essential to the process as conductivity between individual faces 14A- 14L may also be provided, for purposes of electroplating, by the portion of coating 18 extending over the upper surfaces of the ribs 12.

In FIGURE 9 there is shown the comolding 27 after a boring operation in which the center portion, together with segments 19, are removed so as to expose the ends of member 20.

Dimples 30, shown in FIGURES 7 and 8, are molded into the reverse side of base 11 at the time of molding by means of mold pins (FIGURE 4, and 6). The dimples 30 serve as drill locating holes in a subsequent operation wherein holes are drilled to intersec conductive members.

Referring now to FIGURE 9, the resulting holes are then tapped and hollow screws 34 inserted. The hollow screws 34 are tinned and, during final assembly of the completed element, lead wires are inserted into the openings and soldered in place to provide good electrical conductivity and mechanical rigidity.

The structure of FIGURE 9 may be used as a commutator element in conjunction with a brush arranged to wipe the surface 33 of the central bore at a constant angular velocity. This will result in the production of short pulses at a constant frequency. FIGURE is a vertical cross-section taken along line 1010.

If it is desired to obtain pulses of longer duration, the exposed contact area may be enlarged by electroplating with the conductive vanes so connected as to serve as the cathode. It is preferable to coat the inner Wall of the bore with a conductive underlayer, such as a silver base lacquer, before electroplating. After a solid metal ring has been formed by electroplating, the ring may be reamed to provide a smooth surface for the wiper. The ring-may be slotted in a number of places toform a plurality of isolated commutator segments each connected d to a conductive member 20. The resulting slots may be filled with insulator plastic to provide an uninterrupted surface for the wiper.

In FIGURE 11, there is shown a section of a potentiometer element which was formed by taking the member shown inFIGURE 9 .and forming a resistive element 35 on the internal surface 33 of the central bore. The resistance element may be formed by any of the conventional techniques as by spraying with aresistive composition, metalizing, or comolding thereto a conductive plastic layer.

Connection to an external circuit is madethrough'takeoif wires 40 and 42, shown soldered to two of the terminals 34. While only two takeoff wires are shown, any:

suitable number may be employed.

Ring member 45 is molded of insulator plastic and is provided with an inner ring 46 of a :highly conductive metal, such as silver. Electrically interconnected wipers 48 and 50 make connection between the takeoff ring 46 and resistance element 3.5. Wire 43 is used to-connect an external circuit to takeoff ring 46. Wipers48 311(1'50 are mounted on spring members 52 and 54 which are in turn supported by an insulator block '56 attached to arotatable shaft 58. For conventional potentiometer applicationthe resistance element 35 may be interrupted-to provideelectrically isolated end portions. The. interruption may be provided by cutting a slot through the element and then filling the slot with insulator plasticso as to permit 360 rotation of the wiper.

The device of FIGURE 11 will frequently be employed with the two terminals serving as the end terminals of the resistance element connected to an external circuit. The unconnected conductive members 20 will serve as equi-potential distribution means. Thus if the voltage be measured along a line close to and parallel to members 20, that is tosay, transverse to the direction of current flow through the resistance element, the voltage measured would be found to be uniform. This results in avery linear potentiometer sincethe existence of voltage gradients across the element is avoided. Frequently current tends to fiow non-uniformly through various parts of'the cross-section of a conducting member, resulting'in a nonuniform voltage gradient. The construction disclosed herein eliminates this difliculty.

What is claimed is:

l. A potentiometer comprising: an insulator plastic member; an elongated composition resistance elementattached to said insulator plasticrmember; a pair of fixed terminals connected to said resistance element 'at longitudinally spaced points for connecting said resistance element to an external circuit 'so that the normal direction of current flow through said element is between said fixed terminals; a plurality of thin :electrical conductive metal sheets embeddedin theinsulator plastic'memberinispaced relationship to each other, the planes of the sheets being transverse to the longitudinal dimension of saidresistance element, with an edge of each of said sheets in electrical contact with-said resistance element along a line transverse to the said normal direction of current fiow;'a wiper arranged to traverse said resistance element in electrical contact therewith; and means to connect said wiper to an external circuit.

'2. The potentiometer of claim 1 wherein said electrical conductive metal sheets have a thickness of from about 0;00.1 to about 0.002".

3. A potentiometer element comprising: an insulator plastic member; an elongated composition resistance element attached to saidinsulator plastic member; a pair of fixed terminals connected to said resistance element at longitudinally spaced points, for connecting saidresistance element to an external circuit so'that the normal direction of current flow through said element is between said'fixed terminals; a plurality of thin electrical conductive metal sheets embedded in the insulator plastic member in spaced relationship to each other, the planes of the sheets being transverse to the longitudinal dimension of said resistance element, with an edge of each of said sheets in electrical contact with said resistance element along a line transverse to the said normal direction of current flow.

4. The potentiometer of claim 3 wherein said electrical conductive metal sheets have a thickness of from about 0.001" to about 0.002".

References Cited in the file of this patent UNITED STATES PATENTS Fischer Jan. 9, 1923 Apple July 1, 1930 Bullinger Nov. 1 ,1932 Marsten et al. Feb. 24, 1953 Charbonneau Mar. 13, 1956 Kohring Aug. 20, 1957 

